Modulation of cellular cholesterol transport and homeostasis by Rab11

Residence of the Nucleotide Sugar Transporter Family Members SLC35F1 and SLC35F6 in the Endosomal/Lysosomal Pathway

The SLC35 (Solute Carrier 35) family members acting as nucleotide sugar transporters are typically localized in the endoplasmic reticulum or Golgi apparatus. It is, therefore, intriguing that some reports document the presence of orphan transporters SLC35F1 and SLC35F6 within the endosomal and lysosomal system. Here, we compared the subcellular distribution of these proteins and found that they are concentrated in separate compartments; i.e., recycling endosomes for SLC35F1 and lysosomes for SLC35F6. Swapping the C-terminal tail of these proteins resulted in a switch of localization, with SLC35F1 being trafficked to lysosomes while SLC35F6 remained in endosomes. This suggested the presence of specific sorting signals in these C-terminal regions. Using site-directed mutagenesis, fluorescence microscopy, and cell surface biotinylation assays, we found that the EQERLL360 signal located in the cytoplasmic tail of human SLC35F6 is involved in its lysosomal sorting (as previously shown for this conserved sequence in mouse SLC35F6), and that SLC35F1 localization in the recycling pathway depends on two YXXΦ-type signals: a Y367KQF sequence facilitates its internalization from the plasma membrane, while a Y392TSL motif prevents its transport to lysosomes, likely by promoting SLC35F1 recycling to the cell surface. Taken together, these results support that some SLC35 members may function at different levels of the endosomal and lysosomal system.

FACI is a novel clathrin adaptor protein 2-binding protein that facilitates low-density lipoprotein endocytosis

BACKGROUND

Cholesterol plays a vital role in multiple physiological processes. Cellular uptake of cholesterol is mediated primarily through endocytosis of low-density lipoprotein (LDL) receptor. New modifiers of this process remain to be characterized. Particularly, the role of fasting- and CREB-H-induced (FACI) protein in cholesterol homeostasis merits further investigation.

METHODS

Interactome profiling by proximity labeling and affinity purification - mass spectrometry was performed. Total internal reflection fluorescence microscopy and confocal immunofluorescence microscopy were used to analyze protein co-localization and interaction. Mutational analysis was carried out to define the domain and residues required for FACI localization and function. Endocytosis was traced by fluorescent cargos. LDL uptake in cultured cells and diet-induced hypercholesterolemia in mice were assessed.

RESULTS

FACI interacted with proteins critically involved in clathrin-mediated endocytosis, vesicle trafficking, and membrane cytoskeleton. FACI localized to clathrin-coated pits (CCP) on plasma membranes. FACI contains a conserved DxxxLI motif, which mediates its binding with the adaptor protein 2 (AP2) complex. Disruption of this motif of FACI abolished its CCP localization but didn't affect its association with plasma membrane. Cholesterol was found to facilitate FACI transport from plasma membrane to endocytic recycling compartment in a clathrin- and cytoskeleton-dependent manner. LDL endocytosis was enhanced in FACI-overexpressed AML12 cells but impaired in FACI-depleted HeLa cells. In vivo study indicated that hepatic FACI overexpression alleviated diet-induced hypercholesterolemia in mice.

CONCLUSIONS

FACI facilitates LDL endocytosis through its interaction with the AP2 complex.

Cholesterol metabolism pathway in autism spectrum disorder: From animal models to clinical observations

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by a persistent impairment of social skills, including aspects of perception, interpretation, and response, combined with restricted and repetitive behavior. ASD is a complex and multifactorial condition, and its etiology could be attributed to genetic and environmental factors. Despite numerous clinical and experimental studies, no etiological factor, biomarker, and specific model of transmission have been consistently associated with ASD. However, an imbalance in cholesterol levels has been observed in many patients, more specifically, a condition of hypocholesterolemia, which seems to be shared between ASD and ASD-related genetic syndromes such as fragile X syndrome (FXS), Rett syndrome (RS), and Smith- Lemli-Opitz (SLO). Furthermore, it is known that alterations in cholesterol levels lead to neuroinflammation, oxidative stress, impaired myelination and synaptogenesis. Thus, the aim of this review is to discuss the cholesterol metabolic pathways in the ASD context, as well as in genetic syndromes related to ASD, through clinical observations and animal models. In fact, SLO, FXS, and RS patients display early behavioral markers of ASD followed by cholesterol disturbances. Several studies have demonstrated the role of cholesterol in psychiatric conditions and how its levels modulate brain neurodevelopment. This review suggests an important relationship between ASD pathology and cholesterol metabolism impairment; thus, some strategies could be raised - at clinical and pre-clinical levels - to explore whether cholesterol metabolism disturbance has a generally adverse effect in exacerbating the symptoms of ASD patients.

Endothelial LRP1-ICD Accelerates Cognition-Associated Alpha-Synuclein Pathology and Neurodegeneration through PARP1 Activation in a Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by progressive loss of dopaminergic neurons and accumulation of misfolded alpha-synuclein (αSyn) into Lewy bodies. In addition to motor impairment, PD commonly presents with cognitive impairment, a non-motor symptom with poor outcome. Cortical αSyn pathology correlates closely with vascular risk factors and vascular degeneration in cognitive impairment. However, how the brain microvasculature regulates αSyn pathology and neurodegeneration remains unclear. Here, we constructed a rapidly progressive PD model by injecting alpha-synuclein preformed fibrils (αSyn PFFs) into the cerebral cortex and striatum. Brain capillaries in mice with cognitive impairment showed a reduction in diameter and length after 6 months, along with string vessel formation. The intracellular domain of low-density lipoprotein receptor-related protein-1 (LRP1-ICD) was upregulated in brain microvascular endothelium. LRP1-ICD promoted αSyn PFF uptake and exacerbated endothelial damage and neuronal apoptosis. Then, we overexpressed LRP1-ICD in brain capillaries using an adeno-associated virus carrying an endothelial-specific promoter. Endothelial LRP1-ICD worsened αSyn PFF-induced vascular damage, αSyn pathology, or neuron death in the cortex and hippocampus, resulting in severe motor and cognitive impairment. LRP1-ICD increased the synthesis of poly(adenosine 5'-diphosphate-ribose) (PAR) in the presence of αSyn PFFs. Inhibition of PAR polymerase 1 (PARP1) prevented vascular-derived injury, as did loss of PARP1 in the endothelium, which was further implicated in endothelial cell proliferation and inflammation. Together, we demonstrate a novel vascular mechanism of cognitive impairment in PD. These findings support a role for endothelial LRP1-ICD/PARP1 in αSyn pathology and neurodegeneration, and provide evidence for vascular protection strategies in PD therapy.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Modulating cholesterol-rich lipid rafts to disrupt influenza A virus infection

Influenza A virus (IAV) is widely disseminated across different species and can cause recurrent epidemics and severe pandemics in humans. During infection, IAV attaches to receptors that are predominantly located in cell membrane regions known as lipid rafts, which are highly enriched in cholesterol and sphingolipids. Following IAV entry into the host cell, uncoating, transcription, and replication of the viral genome occur, after which newly synthesized viral proteins and genomes are delivered to lipid rafts for assembly prior to viral budding from the cell. Moreover, during budding, IAV acquires an envelope with embedded cholesterol from the host cell membrane, and it is known that decreased cholesterol levels on IAV virions reduce infectivity. Statins are commonly used to inhibit cholesterol synthesis for preventing cardiovascular diseases, and several studies have investigated whether such inhibition can block IAV infection and propagation, as well as modulate the host immune response to IAV. Taken together, current research suggests that there may be a role for statins in countering IAV infections and modulating the host immune response to prevent or mitigate cytokine storms, and further investigation into this is warranted.

Linking Late Endosomal Cholesterol with Cancer Progression and Anticancer Drug Resistance

Cancer cells undergo drastic metabolic adaptions to cover increased bioenergetic needs, contributing to resistance to therapies. This includes a higher demand for cholesterol, which often coincides with elevated cholesterol uptake from low-density lipoproteins (LDL) and overexpression of the LDL receptor in many cancers. This implies the need for cancer cells to accommodate an increased delivery of LDL along the endocytic pathway to late endosomes/lysosomes (LE/Lys), providing a rapid and effective distribution of LDL-derived cholesterol from LE/Lys to other organelles for cholesterol to foster cancer growth and spread. LDL-cholesterol exported from LE/Lys is facilitated by Niemann–Pick Type C1/2 (NPC1/2) proteins, members of the steroidogenic acute regulatory-related lipid transfer domain (StARD) and oxysterol-binding protein (OSBP) families. In addition, lysosomal membrane proteins, small Rab GTPases as well as scaffolding proteins, including annexin A6 (AnxA6), contribute to regulating cholesterol egress from LE/Lys. Here, we summarize current knowledge that links upregulated activity and expression of cholesterol transporters and related proteins in LE/Lys with cancer growth, progression and treatment outcomes. Several mechanisms on how cellular distribution of LDL-derived cholesterol from LE/Lys influences cancer cell behavior are reviewed, some of those providing opportunities for treatment strategies to reduce cancer progression and anticancer drug resistance.

Hydroxycholesterol substitution in ionizable lipid nanoparticles for mRNA delivery to T cells

Delivery of nucleic acids, such as mRNA, to immune cells has become a major focus in the past decade with ionizable lipid nanoparticles (LNPs) emerging as a clinically-validated delivery platform. LNPs-typically composed of ionizable lipids, cholesterol, phospholipids, and polyethylene glycol lipids -have been designed and optimized for a variety of applications including cancer therapies, vaccines, and gene editing. However, LNPs have only recently been investigated for delivery to T cells, which has various therapeutic applications including the engineering of T cell immunotherapies. While several LNP formulations have been evaluated for mRNA delivery, recent work has demonstrated that the utilization of cholesterol analogs may enhance mRNA delivery. Other studies have shown that cholesterols modified with hydroxyl groups can alter endocytic recycling mechanisms. Here, we engineered a library of LNPs incorporating hydroxycholesterols to evaluate their impact on mRNA delivery to T cells by leveraging endosomal trafficking mechanisms. Substitution of 25% and 50% 7α-hydroxycholesterol for cholesterol in LNPs enhanced mRNA delivery to primary human T cells ex vivo by 1.8-fold and 2.0-fold, respectively. Investigation of endosomal trafficking revealed that these modifications also increase late endosome production and reduce the presence of recycling endosomes. These results suggest that hydroxyl modification of cholesterol molecules incorporated into LNP formulations provides a mechanism for improving delivery of nucleic acid cargo to T cells for a range of immunotherapy applications.

Endosomal Cholesterol in Viral Infections - A Common Denominator?

Cholesterol has gained tremendous attention as an essential lipid in the life cycle of virtually all viruses. These seem to have developed manifold strategies to modulate the cholesterol metabolism to the side of lipid uptake and de novo synthesis. In turn, affecting the cholesterol homeostasis has emerged as novel broad-spectrum antiviral strategy. On the other hand, the innate immune system is similarly regulated by the lipid and stimulated by its derivatives. This certainly requires attention in the design of antiviral strategies aiming to decrease cellular cholesterol, as evidence accumulates that withdrawal of cholesterol hampers innate immunity. Secondly, there are exceptions to the rule of the abovementioned virus-induced metabolic shift toward cholesterol anabolism. It therefore is of interest to dissect underlying regulatory mechanisms, which we aimed for in this minireview. We further collected evidence for intracellular cholesterol concentrations being less important in viral life cycles as compared to the spatial distribution of the lipid. Various routes of cholesterol trafficking were found to be hijacked in viral infections with respect to organelle-endosome contact sites mediating cholesterol shuttling. Thus, re-distribution of cellular cholesterol in the context of viral infections requires more attention in ongoing research. As a final aim, a pan-antiviral treatment could be found just within the transport and re-adjustment of local cholesterol concentrations. Thus, we aimed to emphasize the importance of the regulatory roles the endosomal system fulfils herein and hope to stimulate research in this field.

Cholesterol transport between cellular membranes: A balancing act between interconnected lipid fluxes

Cholesterol represents the most abundant single lipid in mammalian cells. How its asymmetric distribution between subcellular membranes is achieved and maintained attracts considerable interest. One of the challenges is that cholesterol rarely is transported alone, but rather is coupled with heterotypic transport and metabolism of other lipids, in particular phosphoinositides, phosphatidylserine, and sphingolipids. This perspective summarizes the major exo- and endocytic cholesterol transport routes and how lipid transfer proteins at membrane contacts and membrane transport intersect along these routes. It discusses the co-transport of cholesterol with other lipids in mammalian cells and reviews emerging evidence related to the physiological relevance of this process.

Non-caveolar caveolins – duties outside the caves

Caveolae are invaginations of the plasma membrane that are remarkably abundant in adipocytes, endothelial cells and muscle. Caveolae provide cells with resources for mechanoprotection, can undergo fission from the plasma membrane and can regulate a variety of signaling pathways. Caveolins are fundamental components of caveolae, but many cells, such as hepatocytes and many neurons, express caveolins without forming distinguishable caveolae. Thus, the function of caveolins goes beyond their roles as caveolar components. The membrane-organizing and -sculpting capacities of caveolins, in combination with their complex intracellular trafficking, might contribute to these additional roles. Furthermore, non-caveolar caveolins can potentially interact with proteins normally excluded from caveolae. Here, we revisit the non-canonical roles of caveolins in a variety of cellular contexts including liver, brain, lymphocytes, cilia and cancer cells, as well as consider insights from invertebrate systems. Non-caveolar caveolins can determine the intracellular fluxes of active lipids, including cholesterol and sphingolipids. Accordingly, caveolins directly or remotely control a plethora of lipid-dependent processes such as the endocytosis of specific cargoes, sorting and transport in endocytic compartments, or different signaling pathways. Indeed, loss-of-function of non-caveolar caveolins might contribute to the common phenotypes and pathologies of caveolin-deficient cells and animals.

[Dynamics of the influenza virus genome regulated by cellular host factors]

For efficient replication of the influenza virus genome and its post-replicational processes, not only viral factors but also host-derived cellular factors (host factors) are required. The influenza virus genome exists as viral ribonucleoprotein (vRNP) complexes with viral RNA-dependent RNA polymerases and nucleoprotein (NP). Using biochemical and proteomics approaches, we have identified host factors which are required for the vRNP replication and the progeny vRNP transport. We found that MCM complex, a cellular DNA replication licensing factor, is required for successful viral genome replication. In concert with the replication reaction, the nascent RNA chains are encapsidated with NP by cellular splicing factor UAP56. Further, after nuclear export of vRNP, we revealed that vRNP is transported to the plasma membrane using cholesterol-enriched recycling endosomes through cell cycle-independent activation of the centrosome by YB-1, which is a mitotic centrosomal protein. Depletion of YB-1 shows that the cholesterol-enriched endosomes are important for clustering of viral structural proteins at lipid rafts to assemble the virus particles concomitantly with the arrival of vRNP beneath the plasma membrane.

The Rab11-binding protein RELCH/KIAA1468 controls intracellular cholesterol distribution

Sobajima et al. identify the novel protein RELCH/KIAA1468 as a Rab11-binding protein and show that RELCH/KIAA1468 and Rab11 regulate OSBP-dependent nonvesicular cholesterol transport from recycling endosomes to the trans-Golgi network.

Cholesterol, which is endocytosed to the late endosome (LE)/lysosome, is delivered to other organelles through vesicular and nonvesicular transport mechanisms. In this study, we discuss a novel mechanism of cholesterol transport from recycling endosomes (REs) to the trans-Golgi network (TGN) through RELCH/KIAA1468, which is newly identified in this study as a Rab11-GTP– and OSBP-binding protein. After treating cells with 25-hydroxycholesterol to induce OSBP relocation from the cytoplasm to the TGN, REs accumulated around the TGN area, but this accumulation was diminished in RELCH- or OSBP-depleted cells. Cholesterol content in the TGN was decreased in Rab11-, RELCH-, and OSBP-depleted cells and increased in the LE/lysosome. According to in vitro reconstitution experiments, RELCH tethers Rab11-bound RE-like and OSBP-bound TGN-like liposomes and promotes OSBP-dependent cholesterol transfer from RE-like to TGN-like liposomes. These data suggest that RELCH promotes nonvesicular cholesterol transport from REs to the TGN through membrane tethering.

Domain 4 (D4) of Perfringolysin O to Visualize Cholesterol in Cellular Membranes-The Update

The cellular membrane of eukaryotes consists of phospholipids, sphingolipids, cholesterol and membrane proteins. Among them, cholesterol is crucial for various cellular events (e.g., signaling, viral/bacterial infection, and membrane trafficking) in addition to its essential role as an ingredient of steroid hormones, vitamin D, and bile acids. From a micro-perspective, at the plasma membrane, recent emerging evidence strongly suggests the existence of lipid nanodomains formed with cholesterol and phospholipids (e.g., sphingomyelin, phosphatidylserine). Thus, it is important to elucidate how cholesterol behaves in membranes and how the behavior of cholesterol is regulated at the molecular level. To elucidate the complexed characteristics of cholesterol in cellular membranes, a couple of useful biosensors that enable us to visualize cholesterol in cellular membranes have been recently developed by utilizing domain 4 (D4) of Perfringolysin O (PFO, theta toxin), a cholesterol-binding toxin. This review highlights the current progress on development of novel cholesterol biosensors that uncover new insights of cholesterol in cellular membranes.

Cholesterol reducing agents inhibit assembly of type I parainfluenza viruses

Many enveloped RNA viruses utilize lipid rafts for the assembly of progeny virions, but the role of cholesterol, a major component of rafts, on paramyxovirus budding and virion formation is controversial. In this study, we analyzed the effects of FDA-approved cholesterol-reducing agents, gemfibrozil and lovastatin, on raft formation and assembly of human parainfluenza virus type 1 (hPIV1) and Sendai virus (SeV). Treatment of the human airway epithelial A549 cells with the agents, especially when combined, significantly decreased production of infectious hPIV1 and SeV. Mechanistic analysis indicated that depletion of cellular cholesterol reduced cell surface accumulation of envelope glycoproteins and association of viral matrix and nucleocapsids with raft membrane, which resulted in impaired virus budding and release from the cells. These results indicate that cellular cholesterol is required for assembly and formation of type 1 parainfluenza viruses and suggest that cholesterol could be an attractive target for antiviral agents against hPIV1.

The lysosomal storage disease continuum with ageing-related neurodegenerative disease

Lysosomal storage diseases and diseases of ageing share many features both at the physiological level and with respect to the mechanisms that underlie disease pathogenesis. Although the exact pathophysiology is not exactly the same, it is astounding how many similar pathways are altered in all of these diseases. The aim of this review is to provide a summary of the shared disease mechanisms, outlining the similarities and differences and how genetics, insight into rare diseases and functional research has changed our perspective on the causes underlying common diseases of ageing. The lysosome should no longer be considered as just the stomach of the cell or as a suicide bag, it has an emerging role in cellular signalling, nutrient sensing and recycling. The lysosome is of fundamental importance in the pathophysiology of diseases of ageing and by comparing against the LSDs we not only identify common pathways but also therapeutic targets so that ultimately more effective treatments can be developed for all neurodegenerative diseases.

Rab11 and phosphoinositides: A synergy of signal transducers in the control of vesicular trafficking

Rab11 and phosphoinositides are signal transducers able to direct the delivery of membrane components to the cell surface. Rab11 is a small GTPase that, by cycling from an active to an inactive state, controls key events of vesicular transport, while phosphoinositides are major determinants of membrane identity, modulating compartmentalized small GTPase function. By sharing common effectors, these two signal transducers synergistically direct vesicular traffic to specific intracellular membranes. This review focuses on the latest advances regarding the mechanisms that ensure the compartmentalized regulation of Rab11 function through its interaction with phosphoinositides.

Clustering of Rab11 vesicles in influenza A virus infected cells creates hotspots containing the 8 viral ribonucleoproteins

Influenza A virus is an important human pathogen causative of yearly epidemics and occasional pandemics. The ability to replicate within the host cell is a determinant of virulence, amplifying viral numbers for host-to-host transmission. This process requires multiple rounds of entering permissive cells, replication, and virion assembly at the plasma membrane, the site of viral budding and release. The assembly of influenza A virus involves packaging of several viral (and host) proteins and of a segmented genome, composed of 8 distinct RNAs in the form of viral ribonucleoproteins (vRNPs). The selective assembly of the 8-segment core remains one of the most interesting unresolved problems in virology. The recycling endosome regulatory GTPase Rab11 was shown to contribute to the process, by transporting vRNPs to the periphery, giving rise to enlarged cytosolic puncta rich in Rab11 and the 8 vRNPs. We recently reported that vRNP hotspots were formed of clustered vesicles harbouring protruding electron-dense structures that resembled vRNPs. Mechanistically, vRNP hotspots were formed as vRNPs outcompeted the cognate effectors of Rab11, the Rab11-Family-Interacting-Proteins (FIPs) for binding, and as a consequence impair recycling sorting at an unknown step. Here, we speculate on the impact that such impairment might have in host immunity, membrane architecture and viral assembly.

LDL-cholesterol transport to the endoplasmic reticulum: current concepts

PURPOSE OF REVIEW

In this article, we summarize the present information related to the export of LDL-derived cholesterol from late endosomes, with a focus on Nieman-Pick disease, type C1 (NPC1) cholesterol delivery toward the endoplasmic reticulum (ER). We review data suggesting that several pathways may operate in parallel, including membrane transport routes and membrane contact sites (MCSs).

RECENT FINDINGS

There is increasing appreciation that MCSs provide an important mechanism for intermembrane lipid transfer. In late endosome-ER contacts, three protein bridges involving oxysterol binding protein related protein (ORP)1L-vesicle associated membrane protein-associated protein (VAP), steroidogenic acute regulatory protein (StAR)D3-VAP and ORP5-NPC1 proteins have been reported. How much they contribute to the flux of LDL-cholesterol to the ER is currently open. Studies for lipid transfer via MCSs have been most advanced in Saccharomyces cerevisiae. Recently, a new sterol-binding protein family conserved between yeast and man was identified. Its members localize at MCSs and were named lipid transfer protein anchored at membrane contact sites (Lam) proteins. In yeast, sterol transfer between the ER and the yeast lysosome may be facilitated by a Lam protein.

SUMMARY

Increasing insights into the role of MCSs in directional sterol delivery between membranes propose that they might provide routes for LDL-cholesterol transfer to the ER. Future work should reveal which specific contacts may operate for this, and how they are controlled by cholesterol homeostatic machineries.

Perfringolysin O Theta Toxin as a Tool to Monitor the Distribution and Inhomogeneity of Cholesterol in Cellular Membranes

Cholesterol is an essential structural component of cellular membranes in eukaryotes. Cholesterol in the exofacial leaflet of the plasma membrane is thought to form membrane nanodomains with sphingolipids and specific proteins. Additionally, cholesterol is found in the intracellular membranes of endosomes and has crucial functions in membrane trafficking. Furthermore, cellular cholesterol homeostasis and regulation of de novo synthesis rely on transport via both vesicular and non-vesicular pathways. Thus, the ability to visualize and detect intracellular cholesterol, especially in the plasma membrane, is critical to understanding the complex biology associated with cholesterol and the nanodomains. Perfringolysin O (PFO) theta toxin is one of the toxins secreted by the anaerobic bacteria Clostridium perfringens and this toxin forms pores in the plasma membrane that causes cell lysis. It is well understood that PFO recognizes and binds to cholesterol in the exofacial leaflets of the plasma membrane, and domain 4 of PFO (D4) is sufficient for the binding of cholesterol. Recent studies have taken advantage of this high-affinity cholesterol-binding domain to create a variety of cholesterol biosensors by using a non-toxic PFO or the D4 in isolation. This review highlights the characteristics and usefulness of, and the principal findings related to, these PFO-derived cholesterol biosensors.

Recycling Endosomes and Viral Infection

Many viruses exploit specific arms of the endomembrane system. The unique composition of each arm prompts the development of remarkably specific interactions between viruses and sub-organelles. This review focuses on the viral–host interactions occurring on the endocytic recycling compartment (ERC), and mediated by its regulatory Ras-related in brain (Rab) GTPase Rab11. This protein regulates trafficking from the ERC and the trans-Golgi network to the plasma membrane. Such transport comprises intricate networks of proteins/lipids operating sequentially from the membrane of origin up to the cell surface. Rab11 is also emerging as a critical factor in an increasing number of infections by major animal viruses, including pathogens that provoke human disease. Understanding the interplay between the ERC and viruses is a milestone in human health. Rab11 has been associated with several steps of the viral lifecycles by unclear processes that use sophisticated diversified host machinery. For this reason, we first explore the state-of-the-art on processes regulating membrane composition and trafficking. Subsequently, this review outlines viral interactions with the ERC, highlighting current knowledge on viral-host binding partners. Finally, using examples from the few mechanistic studies available we emphasize how ERC functions are adjusted during infection to remodel cytoskeleton dynamics, innate immunity and membrane composition.

Annexin A6 and Late Endosomal Cholesterol Modulate Integrin Recycling and Cell Migration

Annexins are a family of proteins that bind to phospholipids in a calcium-dependent manner. Earlier studies implicated annexin A6 (AnxA6) to inhibit secretion and participate in the organization of the extracellular matrix. We recently showed that elevated AnxA6 levels significantly reduced secretion of the extracellular matrix protein fibronectin (FN). Because FN is directly linked to the ability of cells to migrate, this prompted us to investigate the role of AnxA6 in cell migration. Up-regulation of AnxA6 in several cell models was associated with reduced cell migration in wound healing, individual cell tracking and three-dimensional migration/invasion assays. The reduced ability of AnxA6-expressing cells to migrate was associated with decreased cell surface expression of αVβ3 and α5β1 integrins, both FN receptors. Mechanistically, we found that elevated AnxA6 levels interfered with syntaxin-6 (Stx6)-dependent recycling of integrins to the cell surface. AnxA6 overexpression caused mislocalization and accumulation of Stx6 and integrins in recycling endosomes, whereas siRNA-mediated AnxA6 knockdown did not modify the trafficking of integrins. Given our recent findings that inhibition of cholesterol export from late endosomes (LEs) inhibits Stx6-dependent integrin recycling and that elevated AnxA6 levels cause LE cholesterol accumulation, we propose that AnxA6 and blockage of LE cholesterol transport are critical for endosomal function required for Stx6-mediated recycling of integrins in cell migration.

Influenza Virus Induces Cholesterol-Enriched Endocytic Recycling Compartments for Budozone Formation via Cell Cycle-Independent Centrosome Maturation

Influenza virus particles are assembled at the plasma membrane in concert with incorporation of the virus genome, but the details of its spatio-temporal regulation are not understood. Here we showed that influenza virus infection induces the assembly of pericentrosomal endocytic recycling compartment (ERC) through the activation of Rab11a GTPase and cell cycle-independent maturation of centrosome by YB-1, a multifunctional protein that is involved in mitotic division, RNA metabolism and tumorigenesis. YB-1 is recruited to the centrosome in infected cells and is required for anchoring microtubules to the centrosome. We also found that viral infection accumulates cholesterol in ERC and is dependent on YB-1. Depletion of YB-1 shows reduced cholesterol-enriched ERC and prevented budozone formation at the plasma membrane. These results suggest that cholesterol in recycling endosomes, which are emanated from ERC, may trigger the virus assembly concomitantly with the packaging of the virus genome. We propose that the virus genome is transported to the plasma membrane by cholesterol-enriched recycling endosomes through cell cycle-independent activation of the centrosome by YB-1.

Influenza virus particles are assembled at the plasma membrane in concert with incorporation of the virus genome, but the details of its spatiotemporal regulation are unknown. We found that the virus genome is transported to the plasma membrane using cholesterol-enriched recycling endosomes through cell cycle-independent activation of the centrosome by recruiting YB-1, which is a mitotic centrosomal protein. We also revealed that the cholesterol-enriched endosomes are important for clustering of viral structural proteins at lipid rafts to assemble the virus particles. These results suggest that local accumulation of cholesterol, via fusion of endosomes to the plasma membrane, is one of the triggers for the virus assembly concomitantly with arrival of the virus genome beneath the plasma membrane.

Modeling the effects of cyclodextrin on intracellular membrane vesicles from Cos-7 cells prepared by sonication and carbonate treatment

Cholesterol has important functions in the organization of membrane structure and this may be mediated via the formation of cholesterol-rich, liquid-ordered membrane microdomains often referred to as lipid rafts. Methyl-beta-cyclodextrin (cyclodextrin) is commonly used in cell biology studies to extract cholesterol and therefore disrupt lipid rafts. However, in this study we reassessed this experimental strategy and investigated the effects of cyclodextrin on the physical properties of sonicated and carbonate-treated intracellular membrane vesicles isolated from Cos-7 fibroblasts. We treated these membranes, which mainly originate from the trans-Golgi network and endosomes, with cyclodextrin and measured the effects on their equilibrium buoyant density, protein content, represented by the palmitoylated protein phosphatidylinositol 4-kinase type IIα, and cholesterol. Despite the reduction in mass stemming from cholesterol removal, the vesicles became denser, indicating a possible large volumetric decrease, and this was confirmed by measurements of hydrodynamic vesicle size. Subsequent mathematical analyses demonstrated that only half of this change in membrane size was attributable to cholesterol loss. Hence, the non-selective desorption properties of cyclodextrin are also involved in membrane size and density changes. These findings may have implications for preceding studies that interpreted cyclodextrin-induced changes to membrane biochemistry in the context of lipid raft disruption without taking into account our finding that cyclodextrin treatment also reduces membrane size.

The cross-talk of LDL-cholesterol with cell motility: insights from the Niemann Pick Type C1 mutation and altered integrin trafficking

Cholesterol is considered indispensible for the recruitment and functioning of integrins in focal adhesions for cell migration. However, the physiological cholesterol pools that control integrin trafficking and focal adhesion assembly remain unclear. Using Niemann Pick Type C1 (NPC) mutant cells, which accumulate Low Density lipoprotein (LDL)-derived cholesterol in late endosomes (LE), several recent studies indicate that LDL-cholesterol has multiple roles in regulating focal adhesion dynamics. Firstly, targeting of endocytosed LDL-cholesterol from LE to focal adhesions controls their formation at the leading edge of migrating cells. Other newly emerging literature suggests that this may be coupled to vesicular transport of integrins, Src kinase and metalloproteases from the LE compartment to focal adhesions. Secondly, our recent work identified LDL-cholesterol as a key factor that determines the distribution and ability of several Soluble NSF Attachment Protein (SNAP) Receptor (SNARE) proteins, key players in vesicle transport, to control integrin trafficking to the cell surface and extracellular matrix (ECM) secretion. Collectively, dietary, genetic and pathological changes in cholesterol metabolism may link with efficiency and speed of integrin and ECM cell surface delivery in metastatic cancer cells. This commentary will summarize how direct and indirect pathways enable LDL-cholesterol to modulate cell motility.

SNAP23/25 and VAMP2 mediate exocytic event of transferrin receptor-containing recycling vesicles

We recently showed that Rab11 is involved not only in formation of recycling vesicles containing the transferrin (Tfn)–transferrin receptor (TfnR) complex at perinuclear recycling endosomes but also in tethering of recycling vesicles to the plasma membrane (PM) in concert with the exocyst tethering complex. We here aimed at identifying SNARE proteins responsible for fusion of Tfn–TfnR-containing recycling vesicles with the PM, downstream of the exocyst. We showed that exocyst subunits, Sec6 and Sec8, can interact with SNAP23 and SNAP25, both of which are PM-localizing Q_bc-SNAREs, and that depletion of SNAP23 and/or SNAP25 in HeLa cells suppresses fusion of Tfn–TfnR-containing vesicles with the PM, leading to accumulation of the vesicles at the cell periphery. We also found that VAMP2, an R-SNARE, is colocalized with endocytosed Tfn on punctate endosomal structures, and that its depletion in HeLa cells suppresses recycling vesicle exocytosis. These observations indicate that fusion of recycling vesicles with the PM downstream of the exocyst is mediated by SNAP23/25 and VAMP2, and provide novel insight into non-neuronal roles of VAMP2 and SNAP25.

LTP-triggered cholesterol redistribution activates Cdc42 and drives AMPA receptor synaptic delivery

Cholesterol acts as a sensor of NMDA receptor activation and as a trigger of downstream signaling by engaging small GTPase activation and AMPA receptor synaptic delivery during long-term potentiation.

Neurotransmitter receptor trafficking during synaptic plasticity requires the concerted action of multiple signaling pathways and the protein transport machinery. However, little is known about the contribution of lipid metabolism during these processes. In this paper, we addressed the question of the role of cholesterol in synaptic changes during long-term potentiation (LTP). We found that N-methyl-d-aspartate–type glutamate receptor (NMDAR) activation during LTP induction leads to a rapid and sustained loss or redistribution of intracellular cholesterol in the neuron. A reduction in cholesterol, in turn, leads to the activation of Cdc42 and the mobilization of GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid–type glutamate receptors (AMPARs) from Rab11-recycling endosomes into the synaptic membrane, leading to synaptic potentiation. This process is accompanied by an increase of NMDAR function and an enhancement of LTP. These results imply that cholesterol acts as a sensor of NMDAR activation and as a trigger of downstream signaling to engage small GTPase (guanosine triphosphatase) activation and AMPAR synaptic delivery during LTP.

Effect of u18666a on beta-glucosidase, sphingomyelinase, and beta-galactosidase activities in astrocytes of young rats

Niemann-Pick type C disease (NPC) is a neurodegenerative genetic disorder caused by accumulation of lipids, especially cholesterol, in the perinuclear space. U18666A is a cholesterol transport-inhibiting agent, being used to mimic NPC, mainly in fibroblasts. The objective of this study was to observe the effect of the drug U18666A, which causes the accumulation of cholesterol in the cytoplasm of astrocytes from newborn rats, on some lysosomal hydrolase activities. Filipin staining and fluorescence microscopy, through CellM software, were used for visualization and quantification of cholesterol. The dose of U18666A that provided the greatest accumulation of cholesterol was that of 0.25 µg/mL in incubation for 48 h. Primary rat astrocytes incubated with the drug (NPC) showed a significantly higher amount of cholesterol than those without U18666A (controls). The measurement of activity of enzymes sphingomyelinase and beta-glucosidase in astrocytes of rats with NPC was significantly lower than that of control astrocytes, which is consistent with the disease in humans. The activity of the enzyme beta-galactosidase showed no significant difference between both groups. We concluded that U18666A appears to be an excellent intracellular cholesterol transport-inhibiting agent affecting some metabolic pathways in astrocytes of young rats, which mimics NPC in these animals. Just like the change in the activity of lysosomal enzymes has been demonstrated, other biochemical parameters of the cell can be tested with this animal model, thus contributing to a better understanding of the disease.

Role of cholesterol in SNARE-mediated trafficking on intracellular membranes

The cell surface delivery of extracellular matrix (ECM) and integrins is fundamental for cell migration in wound healing and during cancer cell metastasis. This process is not only driven by several soluble NSF attachment protein (SNAP) receptor (SNARE) proteins, which are key players in vesicle transport at the cell surface and intracellular compartments, but is also tightly modulated by cholesterol. Cholesterol-sensitive SNAREs at the cell surface are relatively well characterized, but it is less well understood how altered cholesterol levels in intracellular compartments impact on SNARE localization and function. Recent insights from structural biology, protein chemistry and cell microscopy have suggested that a subset of the SNAREs engaged in exocytic and retrograde pathways dynamically 'sense' cholesterol levels in the Golgi and endosomal membranes. Hence, the transport routes that modulate cellular cholesterol distribution appear to trigger not only a change in the location and functioning of SNAREs at the cell surface but also in endomembranes. In this Commentary, we will discuss how disrupted cholesterol transport through the Golgi and endosomal compartments ultimately controls SNARE-mediated delivery of ECM and integrins to the cell surface and, consequently, cell migration.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Involvement of microtubular network and its motors in productive endocytic trafficking of mouse polyomavirus

Infection of non-enveloped polyomaviruses depends on an intact microtubular network. Here we focus on mouse polyomavirus (MPyV). We show that the dynamics of MPyV cytoplasmic transport reflects the characteristics of microtubular motor-driven transport with bi-directional saltatory movements. In cells treated with microtubule-disrupting agents, localization of MPyV was significantly perturbed, the virus was retained at the cell periphery, mostly within membrane structures resembling multicaveolar complexes, and at later times post-infection, only a fraction of the virus was found in Rab7-positive endosomes and multivesicular bodies. Inhibition of cytoplasmic dynein-based motility by overexpression of dynamitin affected perinuclear translocation of the virus, delivery of virions to the ER and substantially reduced the numbers of infected cells, while overexpression of dominant-negative form of kinesin-1 or kinesin-2 had no significant impact on virus localization and infectivity. We also found that transport along microtubules was important for MPyV-containing endosome sequential acquisition of Rab5, Rab7 and Rab11 GTPases. However, in contrast to dominant-negative mutant of Rab7 (T22N), overexpression of dominant-negative mutant Rab11 (S25N) did not affect the virus infectivity. Altogether, our study revealed that MPyV cytoplasmic trafficking leading to productive infection bypasses recycling endosomes, does not require the function of kinesin-1 and kinesin-2, but depends on functional dynein-mediated transport along microtubules for translocation of the virions from peripheral, often caveolin-positive compartments to late endosomes and ER – a prerequisite for efficient delivery of the viral genome to the nucleus.

Cholesterol regulates Syntaxin 6 trafficking at trans-Golgi network endosomal boundaries

Inhibition of cholesterol export from late endosomes causes cellular cholesterol imbalance, including cholesterol depletion in the trans-Golgi network (TGN). Here, using Chinese hamster ovary (CHO) Niemann-Pick type C1 (NPC1) mutant cell lines and human NPC1 mutant fibroblasts, we show that altered cholesterol levels at the TGN/endosome boundaries trigger Syntaxin 6 (Stx6) accumulation into VAMP3, transferrin, and Rab11-positive recycling endosomes (REs). This increases Stx6/VAMP3 interaction and interferes with the recycling of αVβ3 and α5β1 integrins and cell migration, possibly in a Stx6-dependent manner. In NPC1 mutant cells, restoration of cholesterol levels in the TGN, but not inhibition of VAMP3, restores the steady-state localization of Stx6 in the TGN. Furthermore, elevation of RE cholesterol is associated with increased amounts of Stx6 in RE. Hence, the fine-tuning of cholesterol levels at the TGN-RE boundaries together with a subset of cholesterol-sensitive SNARE proteins may play a regulatory role in cell migration and invasion.

Divergent outcomes following transcytosis of IgG targeting intracellular and extracellular chlamydial antigens

Antibodies can have a protective but non-essential role in natural chlamydial infections dependent on antigen specificity and antibody isotype. IgG is the dominant antibody in both male and female reproductive tract mucosal secretions, and is bi-directionally trafficked across epithelia by the neonatal Fc receptor (FcRn). Using pH-polarized epididymal epithelia grown on Transwells, IgG specifically targeted at an extracellular chlamydial antigen; the major outer membrane protein (MOMP), enhanced uptake and translocation of infection at pH 6-6.5 but not at neutral pH. This was dependent on FcRn expression. Conversely, FcRn-mediated transport of IgG targeting the intracellular chlamydial inclusion membrane protein A (IncA), induced aberrant inclusion morphology, recruited autophagic proteins independent of lysosomes and significantly reduced infection. Challenge of female mice with MOMP-specific IgG-opsonized Chlamydia muridarum delayed infection clearance but exacerbated oviduct occlusion. In male mice, MOMP-IgG elicited by immunization afforded no protection against testicular chlamydial infection, whereas the transcytosis of IncA-IgG significantly reduced testicular chlamydial burden. Together these data show that the protective and pathological effects of IgG are dependent on FcRn-mediated transport as well as the specificity of IgG for intracellular or extracellular antigens.

Raft-like membranes from the trans-Golgi network and endosomal compartments

This article describes a procedure to prepare a raft-like intracellular membrane fraction enriched for the trans-Golgi network (TGN) and endosomal compartments. The initial step in this technique involves cell disruption by homogenization, followed by clearance of the plasma membrane, late endosomes, mitochondria and the endoplasmic reticulum by differential sedimentation. Carbonate treatment, sonication and sucrose density-gradient ultracentrifugation are subsequently used to isolate the target membranes. The isolated subcellular fraction contains less than 1% of the total cellular proteins, but it is highly enriched for syntaxin-6 and Rab11. Typically, 40-60% of the cellular pool of GM1 glycosphingolipid and 10-20% of the total cellular cholesterol cofractionate with this buoyant membrane fraction. Given the role of GM1 as a cell-surface receptor for the cholera toxin and that levels of both GM1 and cholesterol in the TGN-endosomal compartment are upregulated in some inherited diseases, this protocol can potentially be applied to the analysis of disease-associated changes to GM1-enriched intracellular membranes. The isolated membranes are very well separated from caveolin-rich domains of the plasma membrane, the TGN and recycling endosomes. The entire protocol can be completed in as little as 1 d.

LDL cholesterol recycles to the plasma membrane via a Rab8a-Myosin5b-actin-dependent membrane transport route

Mammalian cells acquire cholesterol, a major membrane constituent, via low-density lipoprotein (LDL) uptake. However, the mechanisms by which LDL cholesterol reaches the plasma membrane (PM) have remained obscure. Here, we applied LDL labeled with BODIPY cholesteryl linoleate to identify this pathway in living cells. The egress of BODIPY cholesterol (BC) from late endosomal (LE) organelles was dependent on acid lipase and Niemann-Pick C1 (NPC1) protein, as for natural cholesterol. We show that NPC1 was needed to recruit Rab8a to BC-containing LEs, and Rab8a enhanced the motility and segregation of BC- and CD63-positive organelles from lysosomes. The BC carriers docked to the cortical actin by a Rab8a- and Myosin5b (Myo5b)-dependent mechanism, typically in the proximity of focal adhesions (FAs). LDL increased the number and dynamics of FAs and stimulated cell migration in an acid lipase, NPC1, and Rab8a-dependent fashion, providing evidence that this cholesterol delivery route to the PM is important for cell movement.

Biogenesis and transport of membrane domains-potential implications in brain pathologies

Lipids in biological membranes show astonishing chemical diversity, but they also show some key conserved structures in different organisms. In addition, some of their biophysical properties have been related to specific functions. In this review, we aim to discuss the role of sphingolipids- and cholesterol-rich micro- and nano-membrane domains (MD) and highlight their pivotal role in lipid-protein clustering processes, vesicle biogenesis and membrane fusion. We further review potential connections between human pathologies and defects in MD biosynthesis, recycling and homeostasis. Brain, which is second only to the adipose tissues in term of lipid abundance, is particularly affected by MD defects which are linked to neurodegenerative disorders. Finally we propose a potential connection between MD and several nutrient-related processes and envision how diet and autophagy could bring insights towards understanding the impact of global lipid homeostasis on human health and disease.

Efficiency of siRNA delivery by lipid nanoparticles is limited by endocytic recycling

Despite substantial efforts to understand the interactions between nanoparticles and cells, the cellular processes that determine the efficiency of intracellular drug delivery remain largely unclear. Here we examined cellular uptake of siRNA delivered in lipid nanoparticles (LNPs) using cellular trafficking probes in combination with automated high-throughput confocal microscopy as well as defined perturbations of cellular pathways paired with systems biology approaches to uncover protein-protein and protein-small molecule interactions. We show that multiple cell signaling effectors are required for initial cellular entry of LNPs through macropinocytosis, including proton pumps, mTOR, and cathepsins. SiRNA delivery is substantially reduced as ≅70% of the internalized siRNA undergoes exocytosis through egress of LNPs from late endosomes/lysosomes. Niemann Pick type C1 (NPC1) is shown to be an important regulator of the major recycling pathways of LNP-delivered siRNAs. NPC1-deficient cells show enhanced cellular retention of LNPs inside late endosomes/lysosomes and increased gene silencing of the target gene. Our data suggests that siRNA delivery efficiency might be improved by designing delivery vehicles that can escape the recycling pathways.

Polymeric immunoglobulin receptor traffics through two distinct apically targeted pathways in primary lacrimal gland acinar cells

The polymeric immunoglobulin receptor (pIgR) mediates transcytosis of dimeric immunoglobulin A (dIgA) and its release into mucosal secretions. The present study reveals the complexity of the trafficking of pIgR to the apical plasma membrane in epithelial cells with exocrine secretory functions; in rabbit lacrimal gland acinar cells (LGACs), trafficking of pIgR involves both the transcytotic pathway and one arm of the regulated secretory pathway. By specifically tracking pIgR endocytosed from the basolateral membrane, we show here that the Rab11a-regulated transcytotic pathway mediates the basal-to-apical transport of pIgR, and that pIgR sorted into the transcytotic pathway does not access the regulated secretory pathway. However, previous work in LGACs expanded in the present study has shown that some pIgR is localized to Rab3D-enriched mature secretory vesicles (SVs). Myosin Vb and myosin Vc motors modulate release of proteins from the Rab11a-regulated transcytotic pathway and the Rab3D-enriched secretory pathway in LGACs, respectively. Confocal fluorescence microscopy and biochemical assays showed that inhibition of myosin Vb and myosin Vc activity by overexpression of their dominant-negative mutants each significantly but differentially impaired aspects of apically targeted pIgR trafficking and secretory component release, suggesting that these motors function to regulate pIgR trafficking in both the transcytotic and exocytotic pathways. Intriguingly, a second mature SV population enriched in Rab27b was devoid of pIgR cargo, suggesting the specialization of Rab3D-enriched mature SVs to carry a particular subset of cargo proteins from the trans-Golgi network to the apical plasma membrane.

Cholesterol dependence of collagen and echovirus 1 trafficking along the novel α2β1 integrin internalization pathway

We have previously shown that soluble collagen and a human pathogen, echovirus 1 (EV1) cluster α2β1 integrin on the plasma membrane and cause their internalization into cytoplasmic endosomes. Here we show that cholesterol plays a major role not only in the uptake of α2β1 integrin and its ligands but also in the formation of α2 integrin-specific multivesicular bodies (α2-MVBs) and virus infection. EV1 infection and α2β1 integrin internalization were totally halted by low amounts of the cholesterol-aggregating drugs filipin or nystatin. Inhibition of cholesterol synthesis and accumulation of lanosterol after ketoconazole treatment inhibited uptake of collagen, virus and clustered integrin, and prevented formation of multivesicular bodies and virus infection. Loading of lipid starved cells with cholesterol increased infection to some extent but could not completely restore EV1 infection to control levels. Cold Triton X-100 treatment did not solubilize the α2-MVBs suggesting, together with cholesterol labeling, that the cytoplasmic endosomes were enriched in detergent-resistant lipids in contrast to αV integrin labeled control endosomes in the clathrin pathway. Cholesterol aggregation leading to increased ion permeability caused a significant reduction in EV1 uncoating in endosomes as judged by sucrose gradient centrifugation and by neutral red-based uncoating assay. In contrast, the replication step was not dependent on cholesterol in contrast to the reports on several other viruses. In conclusion, our results showed that the integrin internalization pathway is dependent on cholesterol for uptake of collagen, EV1 and integrin, for maturation of endosomal structures and for promoting EV1 uncoating. The results thus provide novel information for developing anti-viral strategies and more insight into collagen and integrin trafficking.

A human genome-wide screen for regulators of clathrin-coated vesicle formation reveals an unexpected role for the V-ATPase

Clathrin-mediated endocytosis is essential for a wide range of cellular functions. We used a multi-step siRNA-based screening strategy to identify regulators of the first step in clathrin-mediated endocytosis, formation of clathrin-coated vesicles (CCVs) at the plasma membrane. A primary genome-wide screen identified 334 hits that caused accumulation of CCV cargo on the cell surface. A secondary screen identified 92 hits that inhibited cargo uptake and/or altered the morphology of clathrin-coated structures. The hits include components of four functional complexes: coat proteins, V-ATPase subunits, spliceosome-associated proteins and acetyltransferase subunits. Electron microscopy revealed that V-ATPase depletion caused the cell to form aberrant non-constricted clathrin-coated structures at the plasma membrane. The V-ATPase-knockdown phenotype was rescued by addition of exogenous cholesterol, indicating that the knockdown blocks clathrin-mediated endocytosis by preventing cholesterol from recycling from endosomes back to the plasma membrane.

Alleviation of seipinopathy-related ER stress by triglyceride storage

Mutations affecting the N-glycosylation site in Berardinelli-Seip lipodystrophy (BSCL)-associated gene BSCL2/seipin lead to a dominantly inherited spastic paraplegia termed seipinopathy. While the loss of function of seipin leads to severe congenital lipodystrophy, the effects of seipin N-glycosylation mutations on lipid balance in the nervous system are unknown. In this study, we show that expression of seipin N-glycosylation mutant N88S led to decreased triglyceride (TG) content in astrocytoma and motor neuron cell lines. This was corrected by supplementation with exogenous oleic acid. Upon oleic acid loading, seipin N88S protein was relocated from the endoplasmic reticulum (ER) to the surface of lipid droplets and this was paralleled by alleviation of ER stress induced by the mutant protein. This effect was not limited to seipin N88S, as oleic acid loading also reduced tunicamycin-induced ER stress in motor neuron cells. Furthermore, both seipin N88S and tunicamycin-induced ER stress were decreased by inhibiting lipolysis, suggesting that lipid droplets protected neuronal cells from ER stress. In developing zebrafish larvae, seipin N88S expression led to TG imbalance and reduced spontaneous free swimming. Importantly, supplementation with exogenous oleic acid reduced ER stress in the zebrafish head and increased fish motility. We propose that the decreased TG content contributes to the pathology induced by seipin N88S, and that rescuing TG levels may provide a novel therapeutic strategy in seipinopathy.

The protozoan pathogen Toxoplasma gondii targets the paracellular pathway to invade the intestinal epithelium

Abstract  Toxoplasma gondii is a ubiquitous parasite found within all mammals and birds worldwide that can cause fatal infections in immunocompromised persons and fetuses. The parasite causes chronic infections by residing in long-living tissues of the muscle and brain. T. gondii infects the host through contaminated meat and water consumption with the gastrointestinal tract (GI tract) being the first point of contact with the host. The mechanisms by which the parasite invades the host through the GI tract are unknown, although it has been suggested that the paracellular pathway is important for parasite dissemination. Studies indicate that epithelial tight junction-associated proteins are affected by T. gondii, although which junctional proteins are affected and the nature of host protein-parasite interactions have not been established. We have uncovered evidence that T. gondii influences the cellular distribution of occludin to transmigrate the intestinal epithelium and suggest how candidate binding partners can be identified.

Tamarindus indica extract alters release of alpha enolase, apolipoprotein A-I, transthyretin and Rab GDP dissociation inhibitor beta from HepG2 cells

BACKGROUND

The plasma cholesterol and triacylglycerol lowering effects of Tamarindus indica extract have been previously described. We have also shown that the methanol extract of T. indica fruit pulp altered the expression of lipid-associated genes including ABCG5 and APOAI in HepG2 cells. In the present study, effects of the same extract on the release of proteins from the cells were investigated using the proteomics approach.

METHODOLOGY/PRINCIPAL FINDINGS

When culture media of HepG2 cells grown in the absence and presence of the methanol extract of T. indica fruit pulp were subjected to 2-dimensional gel electrophoresis, the expression of seven proteins was found to be significantly different (p<0.03125). Five of the spots were subsequently identified as alpha enolase (ENO1), transthyretin (TTR), apolipoprotein A-I (ApoA-I; two isoforms), and rab GDP dissociation inhibitor beta (GDI-2). A functional network of lipid metabolism, molecular transport and small molecule biochemistry that interconnects the three latter proteins with the interactomes was identified using the Ingenuity Pathways Analysis software.

CONCLUSION/SIGNIFICANCE

The methanol extract of T. indica fruit pulp altered the release of ENO1, ApoA-I, TTR and GDI-2 from HepG2 cells. Our results provide support on the effect of T. indica extract on cellular lipid metabolism, particularly that of cholesterol.

Rab11 regulates exocytosis of recycling vesicles at the plasma membrane

Rab11 is known to associate primarily with perinuclear recycling endosomes and regulate recycling of endocytosed proteins. However, the recycling step in which Rab11 participates remains unknown. We show here that, in addition to causing tubulation of recycling endosomes, Rab11 depletion gives rise to accumulation of recycling carriers containing endocytosed transferrin and transferrin receptor beneath the plasma membrane. We also show that the carriers are transported from perinuclear recycling endosomes to the cell periphery along microtubules. Total internal reflection fluorescence microscopy of cells expressing EGFP-tagged transferrin receptor revealed that Rab11 depletion inhibits tethering and fusion of recycling carriers to the plasma membrane. Depletion of Sec15, which interacts with Rab11, or Exo70, both components of the exocyst tethering complex, leads to essentially the same phenotypes as those of Rab11 depletion. Thus, in addition to its role in recycling processes at perinuclear recycling endosomes, Rab11 is transported along microtubules to the cell periphery through association with recycling carriers, and directly regulates vesicle exocytosis at the plasma membrane in concert with the exocyst.